Various silver salts are utilized in the development of photographic films and papers. These salts, dissolved or suspended in a liquid, are progressively depleted and replenished during well-known image fixing steps carried out in a largely mechanized fashion. Excess silver-bearing liquid is drained into waste holding tanks for later disposal.
Most sewage treatment agencies consider silver suspended in photographic and other liquids to be a hazardous material. Therefore, silver-bearing liquids must be treated to remove substantially all of their silver content before the liquid may be discharged into municipal sewage treatment systems. The concentration of silver permitted to remain in the discharged liquid varies, but generally must fall below 5 parts per million. Many municipalities, nevertheless, require a lower concentration of silver in a liquid destined for sewer disposal.
A wide variety of apparatus have been developed to remove silver from silver bearing liquids so as to permit disposal into a municipal sewer system. It is now standard in the photographic processing industry, however, to use a mass of steel wool or screen material positioned in a canister to induce ion exchange reactions that precipitate silver from silver bearing liquids supplied to the canister. Unfortunately, these canisters have not been altogether satisfactory during use.
Due to the relatively low concentrations of silver found in the liquids typically processed in canisters such as those described above, ion exchange reactions occur at a relatively slow rate. Thus, controlling the amount of time that a silver bearing liquid is in contact with the ferrous ion source is of critical importance. This contact time has, however, been difficult to regulate as degradation of the ferrous ion source during use often leads to wide variations in liquid flow rates through the canister. Total fluid blockages, causing uncontrolled spills of silver bearing liquid, also occur from time to time. A need, therefore, exists for a silver recovery unit capable of effectively recovering silver from a silver bearing liquid in a controlled manner.